Project Summary/Abstract This proposal is for a Limited Competition R03 (Small grant program for NIDDK K23 Recipients). Despite the importance of adoption and maintenance of habitual moderate to vigorous-intensity physical activity (MVPA) for health benefits and long-term weight management, current comprehensive lifestyle interventions place little emphasis on physical activity behavior change strategies and have only modest impact on MVPA outcomes. A smaller scale intervention targeting robust theoretical constructs using novel behavior change strategies to better facilitate adoption and maintenance of MVPA has the potential to make a significant impact on long-term MVPA among inactive adults, but also to inform treatment development of larger, comprehensive lifestyle interventions. The overall goal of this project is to develop and test the feasibility, acceptability, and preliminary efficacy of a workshop intervention designed to increase MVPA among insufficiently active adults who are overweight or obese. All participants will first receive a one-time, 4-hour workshop followed by weekly emails and monthly phone calls for a total of 3 months. The intervention will target a robust theoretical mechanism of change- autonomous motivation- using novel behavior change strategies from Acceptance and Commitment Therapy (ACT). Assessments will be at baseline, 3, and 6 months. We will demonstrate feasibility and acceptability of the ACT workshop intervention via high intervention completion, strategy usage, study retention, and questionnaire ratings of likeability and usefulness. The intervention will be deemed minimally efficacious if we achieve a mean of ?150 minutes of objectively measured bout-related MVPA at study endpoint (6 months). The primary site for this award is the Weight Control and Diabetes Research Center (WCDRC). The WCDRC is part of the Centers for Behavioral and Preventive Medicine at The Miriam Hospital in Providence, Rhode Island, and the Department of Psychiatry and Human Behavior at the Alpert Medical School of Brown University. The proposed ACT workshop intervention is significant because it could produce high, sustained levels of MVPA in a brief, workshop format.